Practice question

XhoI, derived from Xanthomonas holcicola, is a Type II restriction enzyme recognizing hexameric palindrome 5'-C^TCGAG-3' and cutting after first base to leave four-base 5' overhang TCGA. Cleavage requires magnesium and produces cohesive ends compatible with SalI, compatible with XhoI-generated fragments, and useful for creating complementary junctions. The enzyme cuts rarely in AT-rich genomes due to GC-rich recognition, making it attractive for constructing expression cassettes, removing promoters, and performing directional cloning where unique sites are needed to maintain reading frame and transcription orientation.